1. Field of the Invention
This invention relates to a burner for a chemical vapour deposition process, in particular for a chemical vapour deposition process for the manufacturing of a glass preform suitable for drawing an optical fibre.
2. Description Of The Related Art
Two typical processes for producing a glass preform suitable for drawing an optical fibre are the OVD (Outside Vapor Deposition) and the VAD (Vapor-phase Axial Deposition). In these techniques, a burner is used to generate a flame and to eject one or more substances that react in the flame to form silica soot (and being thereofore called “silica precursors”), which is deposited onto a rotating mandrel.
Conventional burners for manufacturing optical preforms comprise a plurality of co-axial ducts, terminating in corresponding nozzles wherein the gases are ejected. During the process, the burner is fed with a silica precursor (such as SiCl4, optionally together with dopants materials, such as GeCl4), combusting gases (e.g. oxygen and hydrogen or methane) and, optionally, some inert gas (e.g. nitrogen, argon or helium). Typically, the glass precursor material is fed through the central duct. The combustible gases comprise a combustible gas containing hydrogen and an auxiliary gas, typically oxygen, that aids the combustible gas to generate a flame. In the gas stream so composed, H2O is produced as a result of the combustion reaction.
When the silica precursor is SiCl4, silica synthesis is achieved through two different physical-chemical processes: a main process of diffusive hydrolysis between the precursor and the combustion product (H2O) by the reaction SiCl4+2H2O→SiO2+4HCl, and a secondary process of premixed oxidation, active at very high temperatures (usually greater than 1000° C.), by the reaction SiCl4+O2→SiO2+2Cl2. An alternative silica precursor is OMTCS (octamethylcyclotetrasiloxane) and premixed oxidation is in this case the only process involved in silica synthesis.
The Applicant has identified, by experimental tests and numeric simulations, two physical phenomena that may limit the complete transformation of the precursor and, as a result, the process yield: penetration of H2O into the SiCl4 stream (or of O2 into the OMTCS stream), and heat penetration into the stream of precursor and forming soot. Reduced heat penetration results, in particular, in a reduced particle heating and in a reduced thermophoretic effect.
The presence of these two phenomena is particularly problematic when a high deposition rate is required. In fact, although an easy way of increasing the reactant flow rate without excessively increasing the exit speed thereof would be to increase the reactant nozzle cross-section (and, accordingly, the burner dimensions), the intense flows produced by such a burner would thwart the above described phenomena, leading to low-efficient processes.
Two burners of a conventional type are described in U.S. Pat. No. 3,565,346 and U.S. Pat. No. 5,922,100.
U.S. Pat. No. 3,565,346 describes a burner having a central circular nozzle for the vaporized silicon tetrachloride, an annular sheath opening surrounding the central nozzle for the exit of an oxygen-containing gas, and a plurality of apertures symmetrically disposed around the annular sheath in a cylindrical configuration, for ejecting a combustible gas.
U.S. Pat. No. 5,922,100 discloses a burner having a central circular nozzle for OMCTS+O2, an annular opening surrounding the central nozzle for ejecting N2, two rings of orifices around the annular opening for ejecting O2, and an outer ring of orifices for ejecting CH4+O2.
It has been observed that these two type of burner can provide good performances only with relatively low reactant flow rates.
JP04-228440 proposes a high-yield multi-flame burner, having the precursor ejecting port of elliptic or rectangular shape, able to improve the deposition yield up to 65% with respect to burners having the precursor ejecting port of circular shape.